A conventional three-axis solid state accelerometer is shown in FIG. 1 of the attached drawings, comprising a proof mass 32, a plurality of sensing beams 42 connecting the proof mass 32 to a proof mass 31 surrounding the proof mass 32. The proof mass 32 is only moveable along y-axis. Electrodes 52p and 52n are located on opposite sides of the proof mass 32 and parallel to x-axis so as to form two y-axis sensing capacitors c92p and c92n. A plurality of sensing beams 41 connects the proof mass 31 to a proof mass 33 surrounding the proof mass 31 so that the proof mass 31 is only moveable along the x-axis. Electrodes 51p and 51n are located on two sides of the proof mass 31 and parallel to the y-axis so as to form two x-axis sensing capacitors c91p and c91n. Two sensing beams 43 connect the proof mass 33 to anchors 60 fixed to boards 71 and 72 so that the proof mass 33 is only moveable along z-axis. Electrodes 93p and 93n are located on the boards 71 and 72 and correspond to the two surfaces of the proof mass 33. The electrodes 93p and 93n form two z-axis sensing capacitors c93p and c93n on two sides of the proof mass 33.
When acceleration occurs in the y-axis, the y-axis sensing capacitors c92p and c92n detect the acceleration due to the change of their capacitances resulting from change of spaces.
When acceleration occurs in the x-axis, the x-axis sensing capacitors c91p and c91n detect the acceleration due to the change of their capacitances resulting from change of spaces.
When acceleration occurs in the z-axis, the z-axis sensing capacitors c93p and c93n detect the acceleration due to the change of their capacitances resulting from change of spaces.
Two deep and narrow spacing parallel surfaces have to be formed when making the x-axis and y-axis capacitors of a conventional accelerometer. The process for making the two deep and narrow spacing surfaces employs surface micro-machining or dry etching, and the aspect ratio is reduced when the depth is increased. This limits the sensitivity of the accelerometer.
The present invention propose to change the structure of the sensing capacitors, when a component of acceleration parallel to the main surface moves the proof masses, the area of the capacitors is changed rather than changing the gap of the capacitors so that the two deep and narrow surfaces are needed. The requirement of the aspect ratio can be omitted so that the process is simplified.